The present invention relates to a novel photosensitive composition and a novel laminated photosensitive element suitable for making a resist for etching or metal plating which is employed, for example, in preparing a printed circuit or in precisely processing a metal article.
In preparing a printed circuit, a resist is employed in an etching or metal plating step. The precision of the resist defines the precision of the etching or metal plating work. When a precise etching or metal plating work is required, a photoresist process being capable of giving a precise resist is employed.
The etching or metal plating employing a photoresist is usually carried out according to the following manner:
(I) A photoresist layer is provided on a surface of a base to be etched or metal plated. The photoresist layer is formed by applying a photosensitive composition solution to the base and drying the obtained coating, or by laminating a so-called non-solvent type photoresist film, which is preformed from the photosensitive composition, to the base.
(II) The photoresist formed on the base is exposed to light through a mask. The exposed areas are thereby hardened and become insoluble in a solvent for the composition.
(III) The unexposed areas are removed by employing a solvent therefor. Conventionally, the procedure is called "development" and the solvent employed is called the "developer".
(IV) The resultant unmasked areas on the base are modified by etching or metal plating.
The main properties required for the photoresist employed in such a process are as follow:
(I) The photoresist should be solid. When a photoresist is exposed to light through a mask, a liquid photoresist is inconvenient to handle and requires a specific exposure device. When a photoresist is formed by applying a photosensitive composition solution to a base, it is required that the photosensitive composition be a solid after drying even though it may be liquid in applying. When a photoresist is formed by laminating a preformed non-solvent type resist film to a base, it is required that the resist film be readily laminated to the base, i.e., that it have good adherence to the base. Since the resist film is laminated with heat and pressure, it is required to have a heat-adhesiveness.
(II) The photoresist should have a good developing property. The solubility of the photoresist to a developer should be remarkably changed by exposure to light. That is to say, it is required that the exposed areas become insoluble in the developer and the unexposed areas remain readily soluble in the developer.
(III) The photoresist should have a good resisting property. The photoresist should protect the masked areas of the base during etching or metal plating. It is required that the photoresist have good adherence to the base as well as good chemical and electrical strengths.
(IV) The photoresist should be readily stripped. The resist is removed after etching or metal plating. Accordingly, the resist should be capable of being readily stripped from the base.
The photosensitive materials employed for preparing the photoresist which are presently sold on the market and employed are classified into the following two groups:
(A) Photocrosslinkable polymer,
(B) A combination of polyfunctional vinyl monomer and unreactive polymer being capable of giving a film-forming property
A representative example of group (A) is polyvinyl cinnamate. This polymer has been employed as a photosensitive material for preparing a photoresist for the longest time. The preparation of a photoresist employing the polymer is carried out only by applying its solution onto a base. A method of employing a preformed non-solvent type photoresist film has not been practiced with the polymer. In order to produce a uniform thick photosensitive layer having no pin-holes, it is necessary for the thickness of the layer to be more than 15 .mu. under the present technique in the art. However, a photosensitive layer of polyvinyl cinnamate having a thickness of more than 15 .mu. cannot be employed because of its low photocrosslinkability, i.e. its low photosensitivity. To the contrary, polymers having an acryloyl group in their side chain are highly photosensitive since their photocrosslinking reaction is radical chain reaction. Accordingly, it seems possible to form a photoresist by employing a preformed non-solvent type photoresist film made of these polymers since there is little limitation relating to the thickness of the photosensitive layer. However, this method has not been practiced because there are probably some problems in the stability of the preformed photoresist film on storage.
In the case of group (B), photocrosslinkability of the polyfunctional vinyl monomer is utilized. Since the photocrosslinking reaction of the polyfunctional vinyl monomer is a radical chain reaction, there is little limitation relating to the thickness of the photosensitive layer. The group (B) photosensitive materials are mainly employed for preparing a preformed non-solvent type photoresist film.
In the case of classifying photoresists, a photoresist produced by employing a photosensitive system containing as the photosensitive material the above-mentioned polyvinyl cinnamate or a polymer having an azide group in its side chain wherein the crosslinking of the polymer is caused by nitrene generated by photolysis of the azide group is generally called a "photocrosslinking type resist", and a photoresist produced by employing a photosensitive system containing a polyfunctional vinyl monomer as the photosensitive material is called a "photopolymerization type resist", taking notice of the utilization of the radical chain photopolymerization reaction of the vinyl group. A photoresist employing the above-mentioned polymer having an acryloyl group in its side chain is classified into the latter, since the polymer is crosslinked but the crosslinking is caused by the radical photopolymerization reaction. The fact that the photoresist is called a photopolymerization type resist does not mean that crosslinking of the polymer does not take place. In other words, a photoresist utilizing the photosensitive system wherein crosslinking takes place but the crosslinking is caused by radical photopolymerization is called a photopolymerization type resist.
It is known that there are various problems with respect to the known photoresists described above. Of these problems, the problem which is desired to be overcome most urgently relates to the stripping property of the photoresist. The known photoresists cause various problems because of the difficulty in stripping them after etching or metal plating.
The problems associated with the stripping of the photoresists formed by employing the conventional photosensitive compositions are as follows:
(1) It is required to employ strong solvents or reagents for stripping the resists. Usually, methylene dichloride or a mixture containing methylene dichloride as the main ingredient together with formic acid and/or methanol is employed. It is difficult to strip the known photoresists by means of 1,1,2-trichloroethylene (trichlene) which is employed in the preparation of a silk screen printing plate. There are two serious problems in the use of methylene dichloride as compared to the use of trichlene. The first is that an etched or metal plated base per se is sometimes damaged in preparing a printed circuit since methylene dichloride is a strong solvent and has a strong permeability. The second is the high volatility of methylene dichloride. Methylene dichloride ruins the working environment since it is very volatile at a normal temperatures. It is difficult to mechanize the stripping operation according to a process of spraying a stripping liquid and circulating it because of the high volatility. It is difficult to obtain a stripping device wherein the escape of methylene dichloride is sufficiently prevented.
(2) Various problems associated with the fact that the known photoresists are stripped swelling-wise. The resist is not completely soluble in the stripping liquid and the stripping thereof is caused mainly by its swelling. Accordingly, in order to remove the resist completely, it is necessary to employ the stripping liquid in combination with a mechanical means such as brushing. However, it is difficult to completely remove a resist having a minute pattern according to this method. Particularly, it is impossible to remove a resist formed on a base that cannot resist the mechanical impact caused by brushing. Since the resist is not dissolved into the stripping liquid but is merely swelled thereby, the debris of the resist stripped is present in the stripping in this manner liquid in a form of particles or membranes. The debris in the stripping liquid causes, in many cases, clogging of spray nozzles or filter media. This defect is another reason why it is difficult to mechanize the operation stripping. There is also a problem of the debris of the resist adhering to the base. Particularly, the debris is liable to adhere to through holes, and it is difficult to remove the debris.
(3) The use of methylene dichloride is economically unfavorable since methylene dichloride per se is more expensive than trichlene and is lost in large amounts during the stripping operation due to its high volativity.
The above-mentioned problems encountered in stripping the known photoresists are substantial problems which are attributable to the photoresists per se. That is to say, since the conventional photoresist technique utilizes a photocrosslinking reaction in order to form a resist pattern corresponding to a mask in a developing step, it is substantially impossible to avoid the above-mentioned difficulties in the stripping operation. However, it is not believed that the photocrosslinking reaction is fundamentally essential for the purpose of forming a resist pattern by utilizing the difference between the solubilities of exposed areas and unexposed areas with respect to a developer in the pattern forming, or developing step. For instance, the use of a monofunctional vinyl monomer instead of a polyfunctional vinyl monomer may be considered. It would be expected that the monofunctional vinyl monomer would be polymerized by photopolymerization to give a polymer which is less soluble than the monomer. In fact, there has been proposed a monofunctional vinyl monomer-photosensitive system even though the purpose thereof is different from that of a photoresist. However, the use of such a photosensitive system for preparing a photoresist is not known. From the historical point of view, the polyfunctional vinyl monomer-photosensitive system has been developed and practiced for the purpose of improving the monofunctional vinyl monomer-photosensitive system (for instance, see Takahiro Tsunoda, "Kankosei Jushi", pages 45 to 50, 121 to 122, and 124; Takashi Tsunoda, "Shikizai", 44(1971), page 68; and U.S. Pat. No. 2,875,047, column 5, lines 15 to 21).
In U.S. Pat. No. 2,760,863, L. Plambeck et al mention the use of compounds having one or more vinyl groups in preparing a printing relief. In the working examples of the patent, however, all photosensitive composition systems employed contain compounds having not less than two vinyl groups and compounds having one vinyl group, and, even though employed, the compounds having one vinyl group are not employed alone but are employed in combination with the compounds having not less than two vinyl groups for the purpose of modifying them. As is clear from the above-mentioned publications, it is generally understood that the achievement of Plambeck et al is to have proposed the polyfunctional vinyl monomer-photosensitive system for the improvement of the monofunctional vinyl monomer-photosensitive system. In Celeste, British Pat. No. 1,275,471, there is exemplified a monofunctional vinyl monomer-photosensitive system which can be employed for a resist. However, the monofunctional vinyl monomer employed in the system is merely employed for the purpose of modification as described in the Plambeck et al patent and the system is photocrosslinked.
From the present inventors' research, it has been determined also that it is impossible to obtain a practical photoresist by employing the photosensitive system which utilizes utilizing only the photopolymerization of a monofunctional vinyl monomer. A first problem is that there does not really exist any monofunctional vinyl monomer having a non-volatility such that only solvents in a resist solution containing the vinyl monomer are selectively removed when a coating of the resist solution is dried to give a solid resist layer. Furthermore, even if a resist containing a volatile monofunctional vinyl monomer as a reactive component is formed, the vinyl monomer would volatilize from the resist with lapse of time. For instance, in the case of a photosensitive system containing N-butoxymethyl acrylamide, which belongs to the group of monomers having the highest boiling points among the known monofunctional vinyl monomers, and acetone, which has a low boiling point as an ordinary solvent, it is difficult to selectively remove only acetone from the coating thereof by drying and it is impossible to control the amount of the monomer remaining in the obtained resist within a desired range after drying. Even a monomer which does not cause the first problem encounters a second problem. This is poor compatibility of the monomer with other components. For instance, monomers such as acrylamide gradually crystallize in the resist and are finally separated out. In the case of N-butoxymethyl acrylamide mentioned above and polypropylene glycol monomethyl ether monomethacrylate, there is no polymer capable of giving a film-forming property which is compatible with them. Accordingly, these monomers migrate onto the surface of the resist with lapse of time and are finally separated out. It is impossible to obtain a photosensitive system suitable for giving a preformed non-solvent type photoresist film for which stability on storage is strongly required, by utilizing conventional monofunctional vinyl monomers in view of the volatility and compatibility problems mentioned above. A third problem is the developer-resistance of the resist. In the resists obtained by employing known monofunctional vinyl monomers, a great difference between the solubilities of the exposed areas and unexposed areas to a developer is not obtained. As a result a faithful resist pattern cannot be obtained. The reasons therefor are that about 12 to about 20% by weight of the monofunctional vinyl monomer (based upon the weight of the resist remains unreacted in the resist) after exposure and that the solubility of the polymer formed by exposure is not much different from that of the original monomer. A fourth problem is the resist property of the resist. The resist obtained from a photosensitive system containing a monofunctional vinyl monomer has a low resistance against etching or a metal plating operation since the resist contains a great amount of unreacted monomer after exposure.
Heretofore, a photosensitive system composed of monofunctional vinyl monomer and polymer capable of giving a film-forming property that does not cause the problems mentioned above has not been known. In the abovementioned U.S. Pat. No. 2,760,863, there are disclosed a variety of monofunctional vinyl monomers (see column 17, line 50, to column 18, line 11). However, the first, second, third and fourth problems mentioned above cannot be avoided with the monomers recited in the patent, not even with styrene which is exemplified as the most preferred example. Even from the viewpoint of only the first and second problems, the recited monomers are unfavorable for giving a preformed non-solvent type photoresist film because of difficulty in the preparation of the film and low stability of the film on storage. Accordingly, these monomers are merely employed as a modifier of the polyfunctional vinyl monomers or an an auxiliary as described in the examples of the patent.
As described above, it is impossible to obtain a photoresist having a superior stripping property according to the prior art.